Republican critics of Hillary Clinton's handling of the Benghazi consulate attack stepped up their pressure on Tuesday ahead of a congressional hearing on the affair.

Lindsey Graham, Republican senator for South Carolina, said the "dam is about to break" on Benghazi and that Clinton, the former secretary of state, would be found to have been "asleep at the wheel".

Testimony due to go before the House oversight and government reform committee on Wednesday is likely to reignite debate over the White House and the State Department's handling of last September's attack on the US consulate in Benghazi. Republican foes of Clinton hope the issue will hamper her widely-rumoured bid for the presidency in 2016.

Gregory Hicks, the former deputy chief of mission in Libya, is due to testify that after an initial assault on the consulate on September 11 last year, a special forces team was blocked from flying from Tripoli to Benghazi by US Special Operations Command Africa (Africom) because it did not have the required authority.

Hicks is expected to add that if the team had been deployed, a second attack on a CIA complex may have been averted. The US ambassador to Libya, Christopher Stephens, died along with three other US officials.

The statements will provide further fodder to Republicans, who have long blamed the State Department for providing inadequate security for the diplomatic mission in Benghazi, and accused the White House of an attempted cover-up over the incident.

It could also serve to further tie Clinton, the former secretary of state, to alleged failings in the incident. Wednesday's testimony before the Republican-led House committee is the latest salvo by political opponents to Clinton over Benghazi, who hope that the issue could hamper a run for president in 2016. In return, Democrats have accused Republican lawmakers of playing politics with an incident in which American lives were lost.

On Tuesday, Graham, a persistent critic of the Obama administration over the Benghazi attack, wrote on his Facebook page: "I think the dam is about to break on Benghazi. We're going to find a system failure before, during, and after the attacks. We're going to find political manipulation seven weeks before an election. We're going to find people asleep at the switch when it comes to the State Department, including Hillary Clinton."

The issue led to angry flare-ups between Clinton and her Republican detractors at a Congressional hearing in January.

In excerpts of pre-hearing testimony by Hicks, the former deputy head of the US mission in Tripoli, he is quoted as saying that a seven-member security team, including two military personnel, flew from Tripoli to Benghazi on news of the first wave of attack.

But a second team, which was preparing to leave the capital city for Benghazi, was told to stand down. "They were told not to board the flight, so they missed it," the Associated Press quoted Hicks as telling Republican committee staff, adding: "I guess they just didn't have the right authority from the right level."

Republicans characterise their refusal to let the Benghazi issue go as a determination to find out what went wrong. But some Democrats have suggested that the real intention is to taint both the White House and Clinton in a bid to dent her chances in 2016, should she decide to run.

State Department spokesman Patrick Ventrell indicated Monday that he believed the committee appeared to be working to political aims. "It certainly seems so, so far," Ventrell said when asked if the department believed the investigation was driven by partisan politics.

Clinton has avoided the political spotlight since stepping down from the State Department in January. Bill Clinton joked about the speculation over her potential 2016 candidacy on Tuesday, insisting that if his wife had made up her mind, she "hasn't mentioned it to me".

Speaking at the 2013 fiscal summit in Washington, Bill Clinton said Hillary was busy working for the Clinton Foundation, writing a book and "having a little fun being a private citizen for the first time in 20 years".